Meta·Software Engineer·Onsite - Coding / Algorithms·Senior

Senior

May 2026

Summary

Meta SWE interview with a matrix manipulation problem that looked straightforward until I actually thought about the overlap case. Tricky in-place constraint with no extra memory allowed.

Questions Asked (1)

Given a 2D matrix, define two equally-sized rectangular regions that may partially overlap. Copy the contents of one region into the other entirely in-place, using no additional memory. Walk through your approach and implement it.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The no-extra-memory part is what tripped me up.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and define the regions precisely, then analyze the overlap to determine the safe copy order. Implement an in-place copy by iterating in the correct direction based on the relative positions of the regions, and test with overlapping and non-overlapping cases.

Pro tip: Mention that this is analogous to memmove and that the key is to avoid overwriting source data before it's copied; demonstrating awareness of memory safety and edge cases shows maturity.

1. Clarify and Define

Ask clarifying questions about matrix dimensions, region sizes, overlap conditions, and whether regions can be identical. Define the regions by their top-left and bottom-right coordinates.

2. Analyze Overlap and Copy Direction

Determine if the regions overlap and their relative positions. Decide the safe iteration order: if the destination is above/left of the source, copy forward; if below/right, copy backward.

3. Implement In-Place Copy

Write nested loops to copy elements from source to destination in the chosen order, ensuring no extra memory is used. Handle row and column offsets correctly.

4. Test and Validate

Test with non-overlapping, partially overlapping, and fully overlapping regions. Verify that the copy is correct and that no data is corrupted.

Key Points to Mention

The problem is equivalent to memmove; overlapping regions require careful copy order.
Copy direction depends on the relative positions of source and destination: forward if dest < src, backward if dest > src.
In-place means O(1) extra space; no temporary arrays or buffers.
Edge cases: identical regions, one region contained in another, non-overlapping regions.
Time complexity is O(area of region), which is optimal.
Use row-major order and adjust indices for rectangular regions.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.